Lapping device

ABSTRACT

A lapping device especially suitable for lapping-in accurate cylindrically convex surfaces comprises an elongated rectangular metal plate having a lapping face on one side and provided on its opposite side with means controlling bending of the plate into a perfectly concave cylindrical axis of curvature by application of bending force to the opposite end portions of the plate. Various means for applying curve-inducing force to the plate are disclosed. A recapable lapping surface for the lapping face of the plate is also provided.

United States Patent 1 Hafner 1 1 LAPPING DEVICE [75] Inventor: Martin Hafner, Munich. Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin and Munich Germany [22] Filed: Aug. 6. 1973 [21] Appl. No.: 386,199

[30] Foreign Application Priority Data Aug. X 1972 Germany 2239073 [52] US. Cl. 51/363; 51/362; 51/325; 51/21 1 [51] Int. Cl 324d 17/00; 824d 37/02 [58] Field of Search 51/362 363, 394. 325. 51/211. 212; 33/176; 144/256; 161/123 [56] References Cited UNITED STATES PATENTS 1.291572 1/1919 Lorenz i 51/362 2374.386 4/1945 Shakesby. 144/256 X 2.508.128 5/1951) Waards i 161/123 X 1.123947 3/1964 Rawley 51/363 [451 Apr. 29, 1975 3.305.976 2/1967 KoSkiC 7. 51/362 3.685216 8/1972 Frey et al 51/325 FOREIGN PATENTS OR APPLICATlONS France 51/212 Primary E.\'aminerA1 Lawrence Smith Attorney. Agent. or FirmHill, Gross, Simpson, Van Santen. Steadman, Chiara & Simpson [57] ABSTRACT A lapping device especially suitable for lapping-in accurate cylindrically convex surfaces comprises an elongated rectangular metal plate having a lapping face on one side and provided on its opposite side with means controlling bending of the plate into a perfectly concave cylindrical axis of curvature by application of bending force to the opposite end portions of the plate. Various means for applying curve-inducing force to the plate are disclosed. A recapable lapping surface for the lapping face of the plate is also provided.

16 Claims. 5 Drawing Figures LAPPING DEVICE This invention relates to lapping devices for producing convex surfaces of cylindrical axis curvature, and is more particularly concerned with producing such convex surfaces which have a large radius of curvature in metallic or ceramic parts.

By way of example, to demonstrate the need for devices of the character to which the present invention relates, information storage and retrieval apparatus in computers, and more particularly the memory heads or cores are referred to, wherein floating shoes of recording heads are related to the memory disks or magnetic storers in such a manner that they float at a slight distance from the opposing disk surface by virtue of the air cushion created by the relative movement of the shoes and rotating disk. For this purpose the floating shoes used are substantially circular or rectangular ceramic plates of approximately mm. diameter with the active, exposed or hearing faces of the floating shoes having a slight convex curvature in the form of a cylindrical section with a shallow radius of curvature of approximately 5 meters.

To produce the convex cylindrical surface of the working part such as a floating shoe, with the required high surface quality, a lapping technique is preferred. Thereby the part is moved with uniform pressure in the presence ofa lapping agent on a lapping plate which for this purpose must have a concave cylindrical axis surface with the radius of curvature desired for the lapped part.

Production of lapping plates with the required exactness of geometric form and surface quality poses difficulties. Suitable machine tools for cutting the proper shape are not available. Attempts to bend a flat uniform thickness plate elastically to form an exact arcuate lapping surface have been unsuccessful due to stress of the plate at the ends and in the middle. Disadvantageously, prior attempts to effect the shaping by bending a plate have'resulted in cross contraction causing bending of the plate normal to the desired bending direction.

An important object of the present invention is to overcome the foregoing and other disadvantages, deficiencies, inefficiencies, shortcomings and problems in the attainment of accurate concave cylindrical axis surfaces on lapping plates.

Another object of the invention is to provide a new and improved lapping device providing a new and improved lapping plate on which an accurate cylindrical axis lapping face is attained easily and simply.

A further object of the invention is to provide a new and improved lapping device providing a lapping plate which can be easily adjusted to provide the desired radius of curvature for a cylindrical axis lapping face.

Still another object of the invention is to provide new and improved means for effecting bending of a lapping plate to provide the desired radius of curvature in a cylindrical axis lapping face of the plate.

Yet another object of the invention is to provide a new and improved method of providing a cylindrical axis lapping face.

Other objects, features and advantages of the invention will be readily apparent from the following de' scription of a preferred embodiment thereof, taken in conjunction with the accompanying drawing although variations and modifications may be effected without departing from the spirit and scope of the novel concepts embodied in the disclosure, and in which:

FIG. 1 is an exploded isometric view showing details of a lapping device embodying features of the invention.

FIG. 2 is a fragmentary side elevational view showing a modified lapping plate supporting and adjusting means.

FIG. 3 is a fragmentary side elevational view of another modified lapping plate supporting and adjusting means.

FIG. 4 is a isometric view showing a modification of the lapping face of the lapping plate.

FIG. 5 is a sectional detail view taken substantially along the line VV of FIG. 1,

On reference to FIG. 1, a representative lapping device is shown including a lapping plate 1 which is preferably of elongated rectangular form and is flat in its normal free condition. In the embodiment shown, which is especially suitable for lapping so called flying brushes or floating shoes for recording heads of computers, the plate 1 may have dimensions of approximately by 200by 9mm. On its upper side the plate provides a lapping face luwhich in the normal unbent condition of the plate 1 is flat. However, for lapping floating shoes, for example, a shallow concave cylindrical axis arcuate shape is necessary for the lapping face 10, such as on a radius of curvature of about Smeters.

According to the present invention attainment of the desired cylindrical concavity of the lapping face laof the plate 1 is attained by applying a bending moment to opposite end margins 2 and 3 of the plate. However, if the plate 1 were in a mere uniform thickness condition, it would be liable to buckling or uneven bending in response to a bending moment applied through the ends 2 and 3 of the plate. To avoid any such buckling or distortions, bending moment is controlled at the opposite side of the plate 1 from the lapping face In. In a preferred means for attaining this end, the opposite or underside of the plate 1 is provided with a set of integral, parallel, coextensive control ribs 4. For example, although dimensioning of the ribs 4 is not critical, excellent results have been obtained in a lapping plate having the aforementioned dimensions by having the ribs of a thickness about equal to their spacing, such as about 4mm. and their width from the plate to their longitudinal edges about 20mm. and with the lengths of the ribs equal to the width of the plate. The rib subdivision should not be too fine or the stability of the ribs becomes too low. However, deviations from the dimensions given for the ribs may be as much as 30%. It will be appreciated, of course, that reasonable adherence to the relative dimensions will afford more uniform results.

To facilitate mounting of the lapping plate 1 for bending moment, it is provided with respective integral transverse stress bars 5 and 6 on the underside of the plate adjacent to the set of ribs 4 and spaced inwardly from the edges of the end margins 2 and 3. Respective connecting pin bores 7 and 8 are provided in the bars 5 and 6, respectively, and are of preferably elongated cross section in a horizontal direction, parallel to the plane of the plate 1.

For supporting the plate 1 and for applying adjustable bending moment thereto, respective complementary members 9 and 10 are provided which may, for convenience, be referred to as jaw meinbers, receptive of the end portions 2 and 3 of the plate. For this purpose, the stress bars and 6 have their respective opposite ends inset relative to the longitudinal side edges of the plate 1 to be received within complementary respective recesses Saand 6ain the members 9 and 10. Thereby pivotal attachment of the bar 5 to the member 9 is readily effected by means of a pin or bolt Shreceived through aligned holes 17 opening into the recess 5aand extending through the bore 7. Similarly, the bar 6 is pivotally connected within the recess 60 by means of a pin or bolt 6breceived through aligned holes 18 opening into the recess 6a and extending through the bore 8. When thus mounted, the plate 1 has its end margins 2 and 3 engaged upon respective upstanding straight across narrow fulcrum edges 15 and 16, respectively. on the members 9 and 10, and of a length preferably equal to at least the width of the plate 1 and engaging as close as practicable to the underside ends adjacent to the end edges of the plate. Thereby, the plate 1 is supported in freely bridging relation clear of and above the members 9 and 10, with the bending bias control ribs 4 extending downwardly freely between the members 9 and 10.

In order to generate the lapping plate bending moment, the members 9 and are mounted for pivotal movement on axes parallel to the plate. To this end, the member 9 is provided with pivot means comprising coaxially aligned trunnions 11 which project from its opposite sides on an axis which lies in a vertical plane which is for best results as nearly as practicable midway between the pivot axes provided by the bolt Sbin the holes 17 and the fulcrum edge 15. Any suitable stand or supporting structure S is provided for pivotally mounting the member 9 through its trunnions 11 engaged in suitable bearings 11a of the supporting structure. Similarly, the member 10 is supported pivotally on a horizontal transverse axis provided by aligned sidewardly projecting trunnions l2 engaging in bearings 12a provided by the supporting structure S. It will be understood of course that the trunnions 12 are counterpart to the trunnions 11 and are located as nearly as practicable in a vertical plane midway between the pivot axes provided by the bolt 6b a and the holes 18a and the fulcrum edge 16.

Adjustable thrust means for effecting equal, pivotal respectively oppositely swinging synchronized adjustment of the members 9 and 10 to effect concave bending moment in the plate 1 comprise a thrust bar 13 extending from one of the members 9 and 10, herein the member 10, toward the member 9 and suitably coupled with a thrust screw 14 (FIGS. 1 and 5). In a simple arrangement, the bar 13 has one end freely engaged in a suitable socket 100 midway between the sides of the block 10 below the recess 6a, and with anti-rotation means in the form of a transverse pin 13a on the end portion of the bar lying in a transverse complementary groove 10b of the pivot block. At its opposite end, the bar 13 extends short of the member 10 and has a concave socket 13b within which a complementary convex tip 14a of the screw 14 engages. Thus, by threadedly manipulating the screw 14 through a threaded bore 9a normally coaxially aligned with the socket 10a, relatively oppositely thrusting force is adapted to be applied to the lower end portions of the pivotally mounted jaw members 9 and 10 for relatively oppositely swinging the members 9 and 10 about their pivot trunnions l1 and 12. This effects leverage on the plate 1 because as the lower end portions of the members 9 and 10 swing outwardly as caused by the equal opposite thrust through thrusting bar 13 and the screw 14, the bolts 5b and 6b are caused to move downwardly and the fulcrum edges 15 and 16 move an equal distance upwardly thereby placing the plate 1 under concave bending stress in the area thereof between the bars 5 and 6 and under the control of the ribs 4. It will be observed that the horizontally cross-sectionally elongated bores 7 and 8 provide lost motion for the bolts 5b and 6b to facilitate the biasing movement and action of the members 9 and 10 relative to the plate 1, any relative adjustments between the edges 15 and 16 and the plate portions 2 and 3, respectively, being accommodated by relative slipping of the engaged surfaces. lf preferred, of course, other means than the bar 13 and screw 14 may be employed to effect controlled plate bending bias swinging of the members 9 and 10 such, for example, as a screw spindle arrangement mounted in universal bearings, toggle linkage and the like.

As will be apparent, the desired concave cylindrical axis bending of the plate 1 and more particularly the lapping surface 10 in the area between the bars 5 and 6 and controlled by the ribs 4 is easily and accurately uniformly attained by the simple elongation of the trust bar and screw assembly 13, 14 applying the members swinging force to their lower ends, and causing the fulcrum edges 15 and 16 to press upwardly on the end extremities of the lapping plate while the bolts 5b and 6b pull down on the plate through the bars 5 and 6. In a lapping plate 1 having the previously mentioned dimensions, the uniformly opposite and relatively large active rotation moments thus provided through the biasing members 9 and 10 enables the bending radius of the plate 1 to be adjusted between a flat condition and a radius of curvature within the elastic limits of the material of the plate 1 which may be to a radius of curvature as small as three meters so that the relatively larger radius of Smeters is easily accomplished within the permissible bending parameters of the plate. By having the rotation axis of the members 9 and 10 as nearly as practicable midway between the points of application of the forces providing the bending moment in the plate, the danger of shearing forces developing in the plate during application of the bending moment is avoided.

Other means may be provided, of course, for effecting the bending moment. For example a very simple arrangement would be to have the bending moment members constructed in such a way that they have a generally bifurcated or fork shaped jaw structure to engage the opposite ends of the lapping plate 1 whereby rotation of the members would effect the bending moment. However such a design of the bending moment means would have the disadvantage that the plateengaging structure would obstruct the lapping surface 1a whereas the arrangement just described in connection with the members 9 and 10 leaves the surface 1a entirely unobstructed.

In the modification of FIG. 2 another means for effecting bending moment is shown in which it will be understood that the lapping plate 1' is substantially the same as the lapping plate 1 of FIG. 1 except to the extent modified as shown in FIG. 2. In this arrangement instead of bars 5 and 6, the plate 1' is provided with an integral bar 20 at each end portion set inwardly from the respective end and provided with a short outwardly projecting leg 21 providing a ledge 210. This provides an upwardly facing fulcrum surface spaced inwardly and downwardly relative to the downwardly facing underside fulcrum surface adjacent to the near end edge of the plate 1'. Bending moment force is effected by means of a pivotal member 22 which has an upwardly projecting fulcrum edge rib 23 to engage the underside of the plate 1 and a downwardly projecting fulcrum edge rib 24 to engage the fulcrum edge Zia. with pivot trunnions 11' located on an axis substantially midway between the fulcrum edges 23 and 24. Through this arrangement. the bending moment members 22 for the lapping plate 1' are adapted to be mounted pivotally in the same manner as the members 9 and 10 and are adapted to be swung about the trunnions 11' by application of force to lower end portions of the members 22 located in advantageous leverage relation at a distance below the plate I and represented by the arrow 13. By swinging the member 22 in Figure 2 toward the right as shown. and its counterpart at the opposite end of the plate I toward the left simultaneously, upward thrust of the fulcrum edge 23 and downward pull effected by the fulcrum edge 24 causes the plate 1' to bend in a uniform manner on a large radius of curvalure.

In FIG. 3 a modified arrangement for applying bending moment. wherein a wider spacing of the fulcrum points is provided for, includes a lapping plate 1" provided with an integral transverse stress bar 25 of an angular cross-sectional profile similar to the stress bar 20 in FIG. 2, but with a short free leg 26 directed inwardly generally away from the end edge of the plate 1" and providing an upwardly facing fulcrum ledge 26a. In this instance. a bending moment jaw member 27 extends upwardly under the associated end portion of the plate I and has a fulcrum ridge or edge ridge 28 engaging the underside of the plate adjacent the end edge to provide upward thrust when the member 27 is pivoted about horizontal axis trunnions 11" by force applied toward the right as indicated by the directional arrow 13". Downward pull on the ledge 26a is effected by the member 27 through a downwardly facing fulcrum rib ridge edge 29 provided on an inward and upward extension portion 270 of the member 27 dimension to clear the ledge flange arm 26 as shown. As will be observed. the pivotal axis provided by the trunnions 11 is located substantially midway between the fulcrum edges 28 and 29. Thus advantageous bending moment leverage is applied to the plate I" by swinging the member 27 in response to force applied as indicated by the arrow 13" toward the right. synchronized with swinging of the counterpart member at the opposite end of the plate 1" toward the left.

During the lapping procedure. the lapping face la of the lapping plate may be used directly and without any intervening surface means as the lapping surface, the lapping compound being applied to such surface. However. such lapping activity causes the lapping face la to become worn and will require replacement of the lapping plate I. That involves a considerable amount of removal and installation time and requires readjustment of the desired concave bending surface radius of curvature. To alleviate that problem. the arrangement shown in FIG. 4 may be adopted. Therein. the lapping plate 1" is in most respects the same as in FIG. 1, although the arrangements of FIGS. 2 and 3 may be employed if preferred. In this instance the plate 1" is provided with the control means ribs 4" is provided with the control means ribs 4" between the stress b and 6". However the plate 1" differs from thr ously described plates in that replaceable lappi face means are provided comprising an under]; 31 and a lapping foil overlayment 32, both of comprise suitable sheet material. If preferred.

derlayment may be omitted and the lapping overlayment 32 may be applied directly to the of the plate 1" provided there are no uneveness as rivets or the like on the plate surface. In any the plate 1" is provided with means to assure conformance of the lapping surface overlaymer sheet 32 to the precise concave contour of tI plate 1". Suction means are provided desirably form of a grid of interconnected crossing grooves 30 in the lapping face of the plate I' s spaced apart and extending at about 45 angle longitudinal sides of the plate. These grooves c nicate in suitable manner with a system of due 33 in the plate. The system of ducts is suitably c with one or more conduits 34 connected with a source (not shown). Where the underlaymen used, it is provided with a pattern of perforati preferably arranged to align with the grooves l security, the underlayment sheet 31 is desirab ably bounded to the face of the plate 1". It will derstood that as shown in FIG. 4 the sheets 31 are only partially illustrated to facilitate desc but will actually be coextensive with the applied the plate 1''. Through this arrangement. the

panel or plate or foil 32 can be readily exchange out requiring any readjustment in the concave I moment of the plate 1".

It will be understood that variations and mi tions may be effected without departing from t1". and scope of the novel concepts of this inventi I claim as my invention:

1. A method of producing a lapping face of I: dius cylindrical curvature, comprising:

providing an elongated lapping plate of resilit terial with a normally flat lapping face; applying bending moment force across the p said face simultaneously to opposite end p of said lapping plate within its elastic limit:

coincident with the application of said bendi ment force controlling bending of the plati area of the plate between said end port cause said lapping face to assume a uniforr drical radius of curvature, comprising con a respective bending moment applying mei each opposite end portion of the plate and s neously actuating said members to thrust u against said plate adjacent to the respective ated end of the plate and to pull down on tl spaced inwardly from said upward thrust ment; and

thrusting said members to swing outwardly in tive opposite directions about respective pi cated substantially midway between the p connection of each of the members.

2. A method according to claim 1, including a to said lapping face a lapping surface sheet, anc ing said lapping surface sheet in conformity w face by vacuum.

3. A method according to claim 2, including 2 a perforated underlayment to said face, effecti uum suction through said underlayment, and r said lapping surface sheet replaceably on said underlayment by said vacuum suction.

4. A lapping device to provide a lapping face of large radius cylindrical curvature. comprising:

an elongated lapping plate of resilient material having a normally flat lapping face;

means for applying bending moment force across the plane of said face simultaneously to opposite end portions of said elongated lapping plate within the elastic limits of the plate;

means in the area of the plate between said end portions for controlling bending of the plate coincident with the application of bending moment force thereto to cause said lapping face to assume a uniform cylindrical radius of curvature throughout said area;

said means for applying bending moment force comprising respective jaw members connected to the opposite end portions of the plate;

said members being connected to said plate to thrust upwardly at the ends of the plate and to pull downwardly on the plate spaced inwardly from said ends; and

means for effecting swinging of said members in respectively opposite outward directions about pivots located substantially midway between the points of connection of the members to the plate.

5. A lapping device according to claim 4. including a lapping surface sheet. and means for maintaining the sheet in conformity with the curvature of the lapping face comprising a vacuum system in the plate.

6. A device according to claim 5, including a perforated underlayment applied directly to said lapping face. and said lapping surface sheet being applied to the underlayment and releasably retained thereon by the vacuum exerted through the underlayment.

7. A lapping device to provide a lapping face of large radius cylindrical curvature. comprising:

an elongated lapping plate of resilient material having a normally flat lapping face;

means for applying bending moment force across the plane of said face simultaneously to opposite end portions of said elongated lapping plate within the elastic limits of the plate;

7 means in the area of the plate between said end portions for controlling bending of the plate coincident with the application of bending moment force thereto to cause said lapping face to assume a uniform cylindrical radius of curvature throughout said area;

said means for applying bending moment force com prising a pair of complementary members supported rotatably on respective axes transverse to the longitudinal axis of the plate and parallel to the plane of said lapping face in its flat state;

said members having fulcrum edges engaging the plate closely adjacent to its respective opposite ends; said axes of the members being located inwardly from said edges and relative to said plate ends;

means spaced inwardly relative to the axes and engaging with the plate to transfer bending forces from the members; and

extensible thrusting means extending to and between the members and operable to rock the members about their axes in respectively opposite outward directions whereby said fulcrum edges thrust upwardly and said force transfer means pull down or the plate to effect said uniform cylindrical radius 0 curvature in said lapping face.

8. A device according to claim 7. wherein the plan has in the area thereof on the opposite side from saic lapping face a set of parallel uniformly dimensionet and spaced transverse ribs providing said means f0 controlling the bending of the plate area. and adjacen to each end of said area a transversely extending stres: bar to which themembers are connected by bolts ex tending through the stress bars with a lost motion con nection.

9. A device according to claim 7. wherein the plan has adjacent to said area respective depending angu larly shaped stress bars providing upwardly facing ful crum ledges engaged by said members.

10. A method of producing a lapping face of large ra dius cylindrical curvature. comprising:

providing an elongated lapping plate of resilient ma terial with a normally flat lapping face;

applying bending moment force simultaneously t oppositeend portions of said lapping plate withir its elastic limits;

coincident with the application of said bending mo ment force controlling bending of the plate in iht area of the plate between said end portions it cause said lapping face to assume a uniform cylin drical radius of curvature;

applying to said lapping face a lapping surface sheet and retaining said lapping surface sheet in conformit with said face by vacuum.

11. A lapping device to provide a lapping face 0 large radius cylindrical curvature. comprising:

an elongated lapping plate of resilient material hav ing a normally flat lapping face: means for applying bending moment force simulta neously to opposite end portions of said elongate lapping plate within the elastic limits of the plate means in the area of the plate between said end por tions for controlling bending of the plate coinci dent with the application of bending moment fore thereto to control bending of the plate to cause sai lapping face to assume a uniform cylindrical radiu of curvature throughout said area;

a lapping surface sheet;

and means for maintaining the sheet in conformit with the curvature of the lapping face comprisin a vacuum system in the plate.

12. A method according to claim 10. including apply ing a perforated underlayment to said face. effectin vacuum suction through said underlayment. and retair ing said lapping surface sheet replaceably on said ur derlayment by said vacuum suction.

13. A device according to claim ll. including a pe forated underlayment applied directly to said lappin face. and said lapping surface sheet being applied to th underlayment and releasably retained thereon by th vacuum exerted through the underlayment.

14. A device according to claim 4, wherein said plat is stiffly resilient. said means for controlling bending said plate area comprise a set of integral transverse ril pa the opposite side of said plate from said lappir ace.

15. A device according to claim 14. comprising r spective stress bars on said plate adjacent to said ril and providing means for connected with said bendir moment force applying jaw members.

16. A device according to claim 4. wherein the la ping plate is a rectangular elongated stiffly resilie metal plate having a plurality of parallel controlling ri on the side opposite said face throughout said area.

i I. t i i 

1. A method of producing a lapping face of large radius cylindrical curvature, comprising: providing an elongated lapping plate of resilient material with a normally flat lapping face; applying bending moment force across the plane of said face simultaneously to opposite end portions of said lapping plate within its elastic limits; coincident with the application of said bending moment force controlling bending of the plate in the area of the plate between said end portions to cause said lapping face to assume a uniform cylindrical radius of curvature, comprising connecting a respective bending moment applying member to each opposite end portion of the plate and simultaneously actuating said members to thrust upwardly against said plate adjacent to the respective associated end of the plate and to pull down on the plate spaced inwardly from said upward thrust engagement; and thrusting said members to swing outwardly in respective opposite directions about respective pivots located substantially midway between the points of connection of each of the members.
 2. A method according to claim 1, including applying to said lapping face a lapping surface sheet, and retaining said lapping surface sheet in conformity with said face by vacuum.
 3. A method according to claim 2, including applying a perforated underlayment to said face, effecting vacuum suction through said underlayment, and retaining said lapping surface sheet replaceably on said underlayment by said vacuum suction.
 4. A lapping device to provide a lapping face of large radius cylindrical curvature, comprising: an elongated lapping plate of resilient material having a normally flat lapping face; means for applying bending moment force across the plane of said face simultaneously to opposite end portions of said elongated lapping plate within the elastic limits of the plate; means in the area of the plate between said end portions for controlling bending of the plate coincident with the application of bending moment force thereto to cause said lapping face to assume a uniform cylindrical radius of curvature throughout said area; said means for applying bending moment force comprising respective jaw members connected to the opposite end portions of the plate; said members being connected to said plate to thrust upwardly at the ends of the plate and to pull downwardly on the plate spaced inwardly from said ends; and means for effecting swinging of said members in respectively opposite outward directions about pivots located substantially midway between the points of connection of the members to the plate.
 5. A lapping device according to claim 4, including a lapping surface sheet, and means for maintaining the sheet in conformity with the curvature of the lapping face comprising a vacuum system in the plate.
 6. A device according to claim 5, including a perforated underlayment applied directly to said lapping face, and said lapping surface sheet being applied to the underlayment and releasably retained thereon by tHe vacuum exerted through the underlayment.
 7. A lapping device to provide a lapping face of large radius cylindrical curvature, comprising: an elongated lapping plate of resilient material having a normally flat lapping face; means for applying bending moment force across the plane of said face simultaneously to opposite end portions of said elongated lapping plate within the elastic limits of the plate; means in the area of the plate between said end portions for controlling bending of the plate coincident with the application of bending moment force thereto to cause said lapping face to assume a uniform cylindrical radius of curvature throughout said area; said means for applying bending moment force comprising a pair of complementary members supported rotatably on respective axes transverse to the longitudinal axis of the plate and parallel to the plane of said lapping face in its flat state; said members having fulcrum edges engaging the plate closely adjacent to its respective opposite ends; said axes of the members being located inwardly from said edges and relative to said plate ends; means spaced inwardly relative to the axes and engaging with the plate to transfer bending forces from the members; and extensible thrusting means extending to and between the members and operable to rock the members about their axes in respectively opposite outward directions whereby said fulcrum edges thrust upwardly and said force transfer means pull down on the plate to effect said uniform cylindrical radius of curvature in said lapping face.
 8. A device according to claim 7, wherein the plate has in the area thereof on the opposite side from said lapping face a set of parallel uniformly dimensioned and spaced transverse ribs providing said means for controlling the bending of the plate area, and adjacent to each end of said area a transversely extending stress bar to which the members are connected by bolts extending through the stress bars with a lost motion connection.
 9. A device according to claim 7, wherein the plate has adjacent to said area respective depending angularly shaped stress bars providing upwardly facing fulcrum ledges engaged by said members.
 10. A method of producing a lapping face of large radius cylindrical curvature, comprising: providing an elongated lapping plate of resilient material with a normally flat lapping face; applying bending moment force simultaneously to opposite end portions of said lapping plate within its elastic limits; coincident with the application of said bending moment force controlling bending of the plate in the area of the plate between said end portions to cause said lapping face to assume a uniform cylindrical radius of curvature; applying to said lapping face a lapping surface sheet; and retaining said lapping surface sheet in conformity with said face by vacuum.
 11. A lapping device to provide a lapping face of large radius cylindrical curvature, comprising: an elongated lapping plate of resilient material having a normally flat lapping face; means for applying bending moment force simultaneously to opposite end portions of said elongated lapping plate within the elastic limits of the plate; means in the area of the plate between said end portions for controlling bending of the plate coincident with the application of bending moment force thereto to control bending of the plate to cause said lapping face to assume a uniform cylindrical radius of curvature throughout said area; a lapping surface sheet; and means for maintaining the sheet in conformity with the curvature of the lapping face comprising a vacuum system in the plate.
 12. A method according to claim 10, including applying a perforated underlayment to said face, effecting vacuum suction through said underlayment, and retaining said lapping surface sheet replaceably on said underlayment by said vacuum suction.
 13. A device according to claim 11, including a perforated underlayment applied directly to said lapping face, and said lapping surface sheet being applied to the underlayment and releasably retained thereon by the vacuum exerted through the underlayment.
 14. A device according to claim 4, wherein said plate is stiffly resilient, said means for controlling bending of said plate area comprise a set of integral transverse ribs on the opposite side of said plate from said lapping face.
 15. A device according to claim 14, comprising respective stress bars on said plate adjacent to said ribs and providing means for connected with said bending moment force applying jaw members.
 16. A device according to claim 4, wherein the lapping plate is a rectangular elongated stiffly resilient metal plate having a plurality of parallel controlling ribs on the side opposite said face throughout said area. 